The nature of a laser beam, and especially its coherent nature, presents a number of problems when used as an illuminating source in applications requiring a uniform illuminating flux over the inspected area, such as is required, for instance, in a wafer inspection system:    (i) Interference of light in the illumination optics creates non-uniformity in the illumination field.    (ii) Interference of the illuminated light by the structured pattern on the wafer creates artifacts in the image.    (iii) Surface roughness creates speckle, that generates non-uniformity in the image.    (iv) The laser beam itself is generally not uniform. Using the laser beam directly as a light source creates non-uniform illumination.
In order to overcome items (i) to (iii) above, the effects of the coherent nature of the laser beam must be reduced and preferably eliminated completely. This process is known as coherence breaking.
There are two definitions related to the coherence of a laser beam:    (a) Spatial coherence, which is the phase relation between each spatial point in the laser beam spot. This allows different points in the spot to interact with each other in a destructive or constructive manner when the spot is illuminating a cyclic pattern or a rough surface. This quality depends mainly on the mode of the beam. For instance in the basic mode (TEM00) the spatial coherence is defined by the Gaussian profile of the beam.    (b) Temporal coherence, which is a measure of the time or the transit distance (the time multiplied by the speed of light in the medium concerned) over which the phase of the beam can be defined. This parameter depends on the type of laser and its spectral bandwidth. Thus, for instance, for the second harmonic of a Nd:YAG laser at 532 nm, the coherence length is about 8 mm in free space.
There are a number of methods described in the prior art for overcoming coherence effects in using laser illumination. Reference is made to the articles “Speckle Reduction” by T. S. McKecknie, pp. 123-170 in Topics in Applied Physics, Vol. 9, Laser Speckle and Related Phenomena, edited by J. C. Dainty, Springer Verlag (1984), “Speckle reduction in pulsed-laser photography” by D. Kohler et al., published in Optics Communications, Vol. 12, No. 1, pp. 24-28, (September 1974) and “Speckle reduction with virtual incoherent laser illumination using modified fiber array” by B. Dingel et al., published in Optik, Vol. 94, No. 3, pp. 132-136, (1993), and to U.S. Pat. No. 6,369,888 to A. Karpol et al., for “Method and Apparatus for Article Inspection including Speckle Reduction”, the disclosures of all of which are herein incorporated by reference, each in its entirety.
The above-mentioned prior art solutions to the problem of coherence breaking variously have specific disadvantages, and it is an object of the present invention to attempt to overcome some of these advantages.